Are the Stingrays (which are useful as a law enforcement tool -- assuming proper warrants are obtained and appropriate restrictions adhered to) ...
There are no proper warrants that can be acquired that can authorize the Constitutional use of a Stingray device, nor are there appropriate restrictions other than a total ban on their use. They are the very definition of blanket surveillance and can not be used in any other way. There is no way to utilize them in a warranty-compliant manner because they will always sweep up the details of everyone in the vicinity, and there is no warrant for that. They are impossible to target, therefore their use by law enforcement (or any private organization being using to whitewash their use by law enforcement) is unconstitutional and therefore illegal.
That's black letter law, too, which is why it's being hidden. There is no sell-us-down-the-river Supreme Court decision that has ruled blanket surveillance legal, unlike, say, the assinine decision that is going to get the 11th Circuit overturned for claiming we have an expectation of privacy for our cell phone records (we do, but the Supreme Court has already ruled, in a massive fit of stupidity, that we don't because the phone company is some sort of magical "third party"). That hasn't happened (yet) with blanket surveillance, and it's hard to imagine even the Roberts court going that far around the bend.
That said, I echo the question you and others posted. How could these devices possibly be so valuable that federal agents are conspiring with local law enforcement to hide their illegal use? I'm assuming they're just unwilling to give up their toys, any toy at all, like the petulant children they are.